Technical Field
Embodiments of the subject matter disclosed herein generally relate to marine seismic sources able to mitigate ghost effect, more specifically, to devices reducing up-going signals that would otherwise cause ghost signals, which interfere with down-going primary signals.
Discussion of the Background
Exploring sedimentary rock formations under the seafloor of the world's oceans and other water bodies is an ongoing process driven by (yet not limited to) seeking gas and oil reservoirs. Variations of seismic signals' propagation velocity from one formation layer to another cause the signals to be reflected, refracted and/or transmitted. Seismic receivers detect time-dependent pressure variations related to the signals traveling through the explored formations. Seismic data (i.e., detected time-dependent pressure variations) provides information about the structure of the explored formation.
Schematically, a marine seismic data acquisition system includes at least one source and one or more receivers. In the marine environment, a source may include plural individual source elements, such as air-guns or vibrators. The source is activated to generate seismic signals (i.e., time-varying pressure propagating in all directions). The receivers may be housed by streamers towed or placed on the seafloor. The sources and/or the receivers are operated to probe formations under the seafloor in a surveyed area.
The seismic signals propagating downward from the source toward the seafloor are known as primary signals. Some of the seismic signals propagating upward from the source to the water surface are reflected at the water-air interface. These reflected signals, known as ghost signals, are phase-shifted about 180°. As is well known, any signal can be expressed as a sum of waves of different frequencies. Having the same source, the ghost and primary signals have similar frequency spectra. The ghost signals are delayed in time (due to the additional path traveled to and from the water surface) and interfere with the primary signals. The resulting interference signals penetrate the seafloor. Some of the waves of the primary and of the ghost signals interfere constructively, but other waves interfere destructively, yielding so-called notches in the spectra of the interference signals. These notches cause loss of information about the explored formation.
Various data acquisition and processing methods try to mitigate the above-described ghosting phenomenon. The methods may require using additional streamers, firing the individual sources following a certain sequence, having individual sources placed at different depths in the water, and/or using time-consuming mathematical algorithms to take into consideration the ghost effect during data processing. However, all these methods result in an undesirable cost increase.
U.S. Pat. No. 8,561,754 (the content of which is incorporated in its entirety herein by reference) describes a source including a coverage (reflection) plate located between individual source elements of a source sub-array and the water surface. The coverage plate reflects less energy than the water-air surface, thus boosting the energy in the low-frequency band of interest. This solution presents the challenge that such a plate made of cement or steel is heavy (e.g., 500-1,000 kg), requiring additional towing energy.
Accordingly, it would be desirable to develop other devices and methods able to mitigate the ghost effect on the signal penetrating the seafloor into the explored formation.